1. Field of the Invention
This invention relates to fiber optic transmission lines and, more particularly, to devices for attachment to optical fibers for demultiplexing signals transmitted therein in both directions.
2. Description of the Prior Art
The field of fiber optics has progressed in a relatively few years from laboratory curiosities and decorative pieces to present-day systems of high sophistication for optical communication and data transmission. Optical fibers or light tubes are specially fabricated filaments which exhibit the property of transmitting light longitudinally along a flexible axis. Various materials which can be used in fabrication of optical fibers and the particular properties thereof are described in Derick et al. U.S. Pat. No. 3,508,589 and, in further detail, in British Pat. No. 1,037,498, cited therein. Low-loss fiber optic taps are important components for fiber optic data links and data buses. This is true because it is desirable to be able to tap a portion of a signal propagating through an optical fiber without breaking or terminating the fiber, since fiber terminations add unwanted optical losses to the system and unfavorably increase the need for highly precise fiber splicing and interconnecting arrangements. Since fiber optic transmission lines having a large number of signal taps are inherently power-starved, it is important to minimize excess losses associated with these components. Furthermore, it is desirable to have taps which can be fabricated so that the tap ratio (the power output of the tap divided by the power into the fiber in a given direction) can be conveniently tailored to the unique requirements of the given system. Efficient fiber optic taps have been reported previously whereby two fibers are cleaved, or ground and polished, at specific angles and butt joined. For example, see Karr et al., "Lightwave Fiber Tap", Applied Optics, Vol. 17, page 2215 (July 15, 1978) and Kuwahara et al., "A Semi-Transparent Mirror-Type Directional Coupler for Optical Fiber Applications", IEEE Transactions on Microwave Theory and Technique, Vol. 23, page 179 (January 1975). In these examples, the tap ratio is variable either by changing the cleavage angle or by using materials with different indices of refraction between the cleaved surfaces. However, devices fabricated by such methods are quite fragile and cannot be easily reproduced with sufficient accuracy.
It has been demonstrated that when an optical fiber is bent in the form of an arc, there is an increased tendency for light to escape from the bent region in a radiation pattern which is primarily in the plane of the bend and which is directed away from the center of curvature. See, for example, Gambling et al., "Radiation From Curved Single-Mode Fibers", Electronics Letters, Vol. 12, page 567 (Oct. 14, 1976); and Goell et al. U.S. Pat. No. 3,982,123. The tendency for light to escape from the bent region of the fibers is enhanced when a flat region is lapped and polished into the fiber surface perpendicular to the radius of the bend in the fiber.
The use of fiber optics to transmit multiplexed signals is also known. U.S. Pat. No. 4,061,577 of Bell mixes multiple signals and transmits them through a fiber optic cable for demultiplexing after conversion to electrical signals by a photodetector. However, this system is quite complex and requires the use of electronics to multiplex and demultiplex the signals. U.S. Pat. No. 3,953,727 of d'Auria et al. both multiplexes and demultiplexes by use of selective mirrors or by lenses and gratings. The signal is transmitted in the multiplexed form using fiber optics after generation by plural diodes and is reflected by selective mirrors to a lens for transmission. A reverse mirror and lens sequence is utilized to reflect the various frequency signals to different photodetectors and a grating is inserted to demultiplex the signal. Thus, d'Auria provides for dual directional transmission of multiplexed signals. In another system, patent 4,155,628 of Schlossberg suggests the use of a series of interferometer elements, each made up of four reflective elements, forming a square optical path, and a pair of beam splitters for input and exit of the signal from the square path. Light enters the square through the first beam splitter, and selectively exits each of the squares through the second splitter.
Further, the use of thin films coupled with prisms is known in the art. For instance, U.S. Pat. No. 3,584,230 of Tien discloses coupling the laser generated light to a thin film waveguide utilizing prisms and lenses. Tien obtains phase matching of the evanescent wave and the wave propagating in the thin film by appropriate alignment of the angle of the input beam. Lastly, dielectric gratings have also been used in thin film waveguides to distinguish between directions of travel. In U.S. Pat. No. 3,982,810 of Tamir et al. a serrated dielectric grating is disposed on the thin film for this purpose.